This invention relates to a device for precluding icing of a cryogenically cooled radiation detector and, more particularly, an X-ray radiation detector operated in a vacuum.
Of interest is commonly owned copending application entitled "Cryogenic Cooling Apparatus for Radiation Detector" Ser. No. 07/862,050 filed concurrently herewith in the name of the present inventors.
Devices for cryogenically cooling radiation detectors are in certain implementations referred to as cold finger assemblies. These assemblies include an elongated finger-like member thermally coupled to a cryogenic Dewar. The Dewar typically is filled with liquid nitrogen for providing cooling of the cold finger in the cold-finger assembly. A radiation detector is thermally conductively attached to an end of the cold finger opposite the cryogenically cooled end. The finger generally is insulated from ambient atmosphere and contained within a housing. The housing may be attached to other systems, for example, in the case of X-ray radiation detection systems, it may be coupled to an electron microscope for windowless X-ray spectroscopic examination of specimens.
In an electron microscope, a cavity of the electron microscope receives a specimen being examined by an electron beam produced by the microscope. The beam incident on the specimen, typically a pencil beam of narrow dimension is incident on the specimen producing X-rays and the resultant X-ray radiation is then radiated from the specimen. The detector is placed within the microscope cavity adjacent to the specimen and detects the radiation radiated from the specimen. The detector converts the radiation to an electrical signal which is passed to an electronic circuit for analysis external the cold finger assembly. The cold finger assembly also includes electronic circuitry for amplifying and sensing the electrical signal produced by the radiation detector.
Some detectors operate in an infrared band and others operate in the X-ray band as discussed above. For example, one such X-ray detector system is disclosed in U.S. Pat. No. 4,931,650. In this environment the known detector comprises a semiconductor mounted at the end of the cold finger or probe introduced into the microscope close to the specimen. The cold finger is surrounded by an envelope and a vacuum is maintained between the finger and the envelope. The cavity in the microscope receiving the cold finger is also at a vacuum. As discussed in this patent, a problem with X-ray detection is that it is sensitive to contamination and, especially, ice buildup. In this patent the prior art is discussed in which the performance of the detector is improved by a warming up procedure. The warming up procedure involves pumping the detector to maintain a vacuum while removing water vapor as it evaporates. Such a procedure is used only as part of a major overhaul involving the return of the detector to the manufacturer. For windowless detectors, a warming up procedure may involve using the pumping system of the microscope. In windowless operation, however, in a spectroscopy type of examination, the microscope cavity and the cold finger may be maintained in an evacuated pressure for many months. Generally prior art systems require that the system be disconnected for a period of time usually every few days or, in some cases, hours, so as to warm up the system and remove the accumulated moisture. The moisture tends to accumulate on the detector decreasing its effectiveness.
As indicated in the aforementioned U.S. Pat. No. 4,931,650, known deicing procedures are time consuming, expensive and may cause contaminated material to be transferred from the detector to the microscope or vice versa during the decontamination procedure. In this patent, conditioning means are provided for locally increasing the temperature of the X-ray detector for a predetermined interval while maintaining a heat sink substantially at the operating temperature and maintaining a physical link in the form of the cold finger between the heat sink and the detector. However, the application of heat, for example by an electrical resistance heater, periodically interrupts the operation of the system and does not remove the problem of ice build up which occurs relatively frequently in terms of long range usage of the microscope system as discussed above.
In U.S. Pat. No. 4,886,240 a non-evacuated Dewar is disclosed which employs a molecular sieve that serves to absorb gases in the Dewar when cooled for operation of a detector to prevent liquid formation onto the detector. A dessicant also may be used to absorb moisture. A Dewar pumping arrangement is disclosed for moisture removal relative to a detector. The detector is secured within the Dewar and includes a lens cap structure. The molecular sieve is employed for removing gases from the area adjacent to the detector when operating. Fluid is contained within the cold finger to expand thereby absorbing thermal energy. The Dewar housing is back filled with inert gas such as nitrogen. This gas is at one atmosphere or atmospheric pressure. However, this system is not disclosed as operating satisfactorily in an evacuated atmosphere especially one in which a microscope is employed in which the microscope emits an electron beam on a specimen to generate X-rays where the specimen needs to be closely spaced to the detector. The use of a lens cap such as disclosed in this patent would not be acceptable in an electron microscope environment. Further, the moisture removing elements might contaminate an evacuated atmosphere.